The title says it all, really. Normally, when one hasn't done anything the "I'm done" option in the First Post review queue is greyed-out. Now, it is no longer so, but one still cannot click it. Similarly, after having done something, the "No Action Needed" button is not greyed-out, yet unclickable. The same happens in e.g. the Late Answers review queue.
I speculate that this bug was introduced with the new font (I noticed it more than one day, but not much more than that, ago), though I have no evidence to support that claim.